In terms of software engineering, legacy systems cause a lot of problems in maintenance. A web application may quickly become legacy due to the fast evolution of web technology. Therefore, a transition method for migrating a web application from old to modern technology is needed. In this paper, a three-phase transition method is proposed. The API of the web application is determined in the first phase. To focus on the API and user interface definition, the concept of adapter service is introduced. The users are encouraged to participate in the prototyping process that refines the API and user interface iteratively. In the second phase, the developers focus on implementing a production-grade web service that conforms to the adapter service. In the third phase, the developers focus on the administrative tasks such as data migration, application deployment, and configuration. To demonstrate the effectiveness of the method, a case study is provided at the end of the paper.